1. Field of the Invention
This invention relates to an improved electrically conductive current collector suitable for use in high temperature applications in the presence of corrosive environments.
More particularly this invention relates to an electronically conductive current collector or current collector/container for use in energy conversion devices such as the sodium-sulfur battery.
Still more particularly this invention relates to two methods for preparing electronically conductive rutile titanium dioxide which is ideally suited for use in the current collector of the invention.
2. Prior Art
There are a number of electrical applications involving various energy conversion devices in which the current collector of the device is exposed to an extremely corrosive environment. For example in energy conversion devices of the type comprising a molten cathodic reactant such as sodium polysulfide, the selection of a suitable current collector as well as a suitable container has been a source of considerable concern.
Previously, one of the prime candidates to date for use as a current collector material or current collector/container for such devices has been certain noncorrosive metals. However, metal systems, both pure and alloyed, often exhibit the phenomenon of severe plastic deformation under stresses. Such stresses are common in energy conversion devices due both to forces applied to the system from external sources and forces arising within the system, including those from expansion and contraction under thermal cycling as the system operates. Furthermore, the high operating temperatures of such systems restricts the use of any metal whose melting point or point of plastic deformation approximates such termperatures, without respect to application of the aforementioned forces. For this reason and because of severe corrosion problems, many metals are not practical for use in such high temperature or corrosive (oxidative) environments.
Since the thermodynamic stability of ceramic materials such as oxides and sulfides in the presence of corrosive environments is well established and since it is also known that the thermodynamic stability of such materials is maintained to temperatures much higher than is compatible for metal systems, it has been suggested to employ a ceramic coating on the metal load bearing element of the aforementioned metallic current collector or container. Where a metal system operates as the load bearing element and includes such a protective covering separating the metal from the corrosive substance, the selection of a suitable covering must be made from materials which (1) are noncorrosive and impermeable to the corrosive substance, (2) adhere well under conditions of thermal cycling and (3) have sufficient electronic conductivity.
Often times a thermal expansion mismatch between the attached metal and ceramic covering results in fractures, microcracks and eventual spalling of the ceramic coating from the metal load bearing member. In addition to the limitations caused by mechanical incompatibility, considerable difficulty in applying the ceramic coating has been experienced. Conventional methods of application such as anodizing, for example, often result in an insulative rather that a conductive coating. In summary, the concurrent development of the requisite noncorrosive character, good adherence, and adequate conductivity in a coating which will be mechanically compatible under recurring cycles of thermal expansion has long presented a difficult challenge in this field of art.
In view of the above discussed inherent limitations of current collecting systems comprising a metal load bearing element with a corrosion resistant ceramic coating, the use of corrosion resistant ceramic per se has been suggested. However, the vast majority of useful ceramics are electrical insulators, thus making them unsuitable for current collection purposes. The utility of ceramics as insulative material is disclosed, for example, in Kummer, et al U.S. Pat. No. 3,404,035, which discusses the use of ceramics as a containing member in an electrical conversion system. This insulative character, in fact is incorporated as an essential element for enabling effective operation of the system (FIG. 1).
Kummer, supra, also discloses the use of a metal covering in combination with insulative ceramic (FIG. 4); however, this application does not relate to the present invention. Kummer's use of a metal covering is necessitated because of the fragile nature of the ceramic chosen for the system, i.e., glass. The metal functions solely as a protective covering for the glass and is not a part of the electronically conductive circuit. Not only does the metal not function as a current collector, but the glass with which it is used is not operable as an electronically conductive medium. Instead, Kummer uses conventional graphite cathode or anode means to close the circuit of the energy conversion device.
A limited class of ceramics are known to be conductive in the metallic sense, but are not economically attractive. A larger class of ceramics can be made moderately conductive, but with conductivities which are much less than metals. Consequently, a current collector constructed of an electronically conductive ceramic of the latter group will exhibit a much higher resistance than that of a similarly shaped metal current collector.
It is therefore an object of this invention to provide a current collector and/or container which possesses the concurrent characteristics of being (1) noncorrosive and impermeable to corrosive substances (2) electronically conductive and (3) mechanically stable when subjected to thermal cycling.